Conventionally, image systems, which uses a reflecting optical elements, such as a flight simulator, head-mounted display, projector, and the like, are known. Among these systems, particularly as a proposal of a reflecting optical element material mainly containing a metal material or a reflecting optical element, a reflecting material in which an Al (aluminum) base metal is continuously deposited on the surface of an Al rolled material, as disclosed in Japanese Patent Laid-Open No. 7-243027, is known. Also, a method of manufacturing a curved surface mirror by processing, particularly a stainless steel plate material, which is mirror-surface polished, in accordance with press forming or bulge forming, as disclosed in Japanese Patent Laid-Open No. 8-36222, is known. In addition, a method of obtaining an illuminating mirror by subjecting Al, an Al alloy, or stainless steel to drawing or dieless forming to have a curved surface and thereafter processing the curved surface by various types of polishing operations, as disclosed in Japanese Patent Laid-Open No. 9-120705, is known.
A conventional metal optical element material has crystal anisotropy parallel to the rolling direction of the material, and its mechanical characteristics differ between the rolling direction of the material and a cross direction to the rolling direction. Because the material is compressed in the direction of plate thickness, the hardness of the material surface is very high. Therefore, when a formed product such as a concave mirror is to be manufactured by pressing, as the transcribing performance of the press die differs between the rolling direction and the direction perpendicular to the rolling direction, it is difficult to obtain high shape precision. Also, as the hardness of the material surface is high, it is difficult to sufficiently transcribe the smoothness of the surface of the press die to the surface of the press formed product.
For these reasons, in most metal reflecting optical elements, as seen in above patent references 1 to 3, polishing and mirror-surface processing are performed to ensure the shape precision and surface roughness, however, this results in long processing times.
A material which focuses only on the surface roughness has also been developed. In the reflecting material disclosed in patent reference 1, which is obtained by continuously depositing the Al base metal on the surface of the Al rolled material, the crystal grain size decreases to improve the surface smoothness. When the crystal grain size is decreased, the surface roughness of the material itself decreases to improve the reflectance. In the depositing step, however, if deposition is performed to decrease the crystal grain size, the deposition layer is hardened due to the Hall-Petch effect. Then, in the press step, the smoothness of the surface of the press die cannot be sufficiently transcribed to the surface of the press formed product, and surface cracks tend to easily occur, making it difficult to form a free-form surface which is smooth and has a small curvature radius.
According to another method, a commercially available expensive optical aluminum material is used. Japanese Patent Laid-Open No. 2002-316226 discloses a method of imparting gloss to a laminate of high-purity aluminum and low-purity aluminum by roller burnishing, and embossing the laminate with a superhigh stress, thus obtaining a metal optical element having a diameter of about 1 mm to 2 mm. With this method, as the material is originally smooth and a high load can be applied to a very small area of the material, a yield strength which is equal to or more than work-hardening of the material can be achieved. However, as the hardness of the material surface is high, the smoothness of the die cannot be sufficiently transcribed, and the resulting surface roughness is as high as 10 nm or more. In addition, in this aluminum optical element, since the original material has strong anisotropy, if the demanded precision of the optical element is to be satisfied, only a small formed product having a diameter of 2 mm or less can be obtained.